In the manufacture of a semiconductor device, a laser irradiation method is often used. One reason for this is that the processing time can be more drastically reduced in the laser irradiation method than in a solid phase crystallization method utilizing radiation heating or conductive heating. Another reason is that a substrate which is easily deformed by heat, such as a glass substrate does not suffer thermal damages in the laser irradiation method.
For conducting laser irradiation, an irradiation object needs to be scanned with a laser beam which has been emitted from a laser oscillator. As a scanning method of a laser beam, there is an optical system using a galvanometer mirror and an f-θ lens. Since the galvanometer mirror has a variable angle of inclination, an arbitrary position on an irradiation object can be irradiated with a laser beam which has been reflected by the galvanometer mirror. This enables laser irradiation over an entire surface of the irradiation object by conducting scan in an X direction with a galvanometer mirror and conducting transition in a Y direction with a robot provided over a stage. Further, when two galvanometer mirrors are disposed, one of them conducts scan in an X direction, and the other conducts scan in a Y direction, an arbitrary position on an irradiation object can be irradiated with a laser beam (for example, Reference 1: Japanese Published Patent Application No. 2003-86507).